1. Field of Invention
The present invention relates generally to the production of dimethylnapthalene. In particular, it relates to a process for preparing 2,6-dimethylnapthalene by dehydrogenation and cyclization of 1-(p-tolyl)-2-methylbutane or 1-(p-tolyl)-2-methylbutene) in the presence of a catalyst.
2. Description of Related Art
2,6-dimethylnapthalene (in the following also abbreviated xe2x80x9c2,6-DMNxe2x80x9d) is a desirable raw material required for the production of 2,6-naphthalene dicarboxylic acid. Said carboxylic acid can be obtained by oxidation using known oxidation processes (see for example U.S. Pat. No. 5,183,933, Harper et al.). 2,6-naphthalene dicarboxylic acid is an important intermediate in the manufacture of speciality polymers, such as poly(ethylene naphthalate) (PEN) and liquid crystal polymers. The cost of PEN depends strongly on the price of 2,6-naphthalene dicarboxylic acid and, thus, on the price of its raw material 2,6-DMN. Therefore, for the economics of the process, it is essential that 2,6-DMN of high purity and good quality is selectively produced with an inexpensive process.
At present, 2,6-DMN is prepared by a multistep synthesis which starts by alkylation of o-, m- or p-xylene with 1- or 2-butene or, preferably, butadiene to yield 5-(o-, m-, or p-tolyl)-pent-1 (or -2-)-ane or -ene, which is subsequently converted to 1,5-, 1,6-, 2,5- or 2,6- dimethyltetralin. The tetralins are dehydrogenated to the corresponding dimethylnaphthalenes, and isomerized to yield 2,6-dimethylnapthalene (2,6-DMN). This prior art is discussed by Sikkenga et al. who disclose multi-step liquid phase syntheses for the cyclisation of specific alkenyl benzenes to one or more specific dimethyl tetralins in the presence of suitable solid acid cyclisation catalysts, such as acidic crystalline zeolites, followed by dehydrogenation to corresponding dimethylnapthalene(s) and isomerization of the resulting dimethylnaphthalenes to the desired specific dimethylnapthalene (cf. U.S. Pat. Nos. 5,073,670, 5,401,892, 5,118,892, 5,012,024 and 5,030,781 and Published PCT Application WO 89/12 612.
A problem associated with most of the prior art methods is the utilisation of l-(o-, m-, or p-tolyl) pent-1-or-2-ene type straight-chained alkenylated compounds as a starting material, which results in the use of an acidic cyclisation catalyst. Consequently, after dehydrogenation the product stream contains several dimethylnaphthalenes, and a subsequent step of isomerisation and/or separation is required. It has therefore been highly desired to improve the selectivity of the cyclisation step in the aforesaid multistep processes, and even more desirable also to decrease the number of necessary process steps in order to achieve a more economic performance.
To that end, EP 0 430 714 B1 (Mitsubishi Gas Chemicals) suggests a process for producing 2,6-dimethylnapthalene by subjecting 2-methyl-1-(p-tolyl)-butene, 2-methyl-1-(p-tolyl)-butane or a mixture of thereof to cyclization and dehydrogenation in the presence of (a) a catalyst comprising lead oxide and/or indium oxide and aluminum oxide; (b) a catalyst comprising lead oxide and/or indium oxide, aluminum oxide and alkali metal oxide and/or alkaline earth metal oxide; (c) a catalyst comprising lead oxide and/or indium oxide, aluminum oxide and at least one oxide of the metals: iron, tin antimony, chromium, zinc, vanadium, nickel or cobalt; or (d) a catalyst comprising lead oxide and/or indium oxide, aluminum oxide, an oxide of iron, tin antimony, chromium, zinc, vanadium, nickel and/or cobalt and an oxide of alkali metal and/or alkaline earth metal. A later patent, EP 0 546 266 B1, discloses the use of a catalyst comprising a platinum component and at least one component selected from alkali metals or alkaline earth metals and supported on alumina. Finally , EP 0 557 722 B1 discloses the use of a catalyst which comprises palladium, alkali metal compound and aluminum oxide. In the catalyst, 0.05-20 wt-% of palladium together with 0.1-20 wt-% of alkali metal is supported on alumina, and the catalyst is used in cyclisation dehydrogenation reaction at 350-700 xc2x0 C. in the presence of a solvent/diluent such as toluene, benzene, steam or the like to suppress side reactions such as polymerisation.
Although the selectivity of the process has been somewhat increased by the art suggested in above-mentioned patents, further improvement is still required. There is also a need for an inexpensive process which provides for easy separation of by-products. In particular the formation of alkyl-substituted indanes and indenes should be minimized.
It is an object of the present invention to provide a process for manufacturing 2,6-DMN with improved yield and selectivity by cyclisation of an alkenylbenzene (alkyl-benzene), 1-(p-tolyl)-2-methylbutene (1-(p-tolyl)-2-methylbutane).
It is a second object of the present invention to provide a novel catalyst which is suitable for use in, e.g., dehydrocyclisation reactions.
It is a third object of the present invention to provide a new use of activated carbon.
These and other objects, together with the advantages thereof over known processes, which shall become apparent from the specification which follows, are accomplished by the invention as hereinafter described and claimed.
The present invention is based on the finding that a key feature in selectivity enhancement of dehydrocyclisation reactions is the composition of the heterogeneous catalyst. By neutralisation of acid sites present on supports of catalysts for dehydrocyclisation it becomes possible to suppress unwanted cracking reactions as well as undesired acid catalysed condensation reactions. Surprisingly, it has further been found that high activity and good selectivity is obtained with inexpensive catalysts consisting essentially of activated carbon. Thus, such materials which traditionally have been used as supports for noble metal catalyst have turned out to possess high activity for dehydrocyclisation as such (without any catalytically active metal species). The activity of the activated carbon is further increased when it is neutralized or otherwise modified so as to produce non-acidic carbon. When a noble metal is deposited on such a support a non-acidic noble metal catalyst is obtained on which the reaction proceeds only on the metallic sites.
The present invention also provides a novel kind of catalyst comprising chromium deposited on an active carbon support. This catalyst is useful for dehydrocyclisation reactions.
The present invention provides considerable advantages. Thus, the selectivity is improved compared to that of bifunctional systems, wherein two types of active sites, the metallic and acidic, operate simultaneously offering diverse reaction routes. Activated carbon is an inexpensive catalyst that is readily available. Furthermore, it has been found that advantageous results are obtained by employing a highly dispersed catalyst for ring closure in dehydrocyclisation. Monofunctional non-acidic well-dispersed noble metal catalysts are therefore suitable for dehydrocyclisation of a specific methyl-alkenylbenzene, such as 1-(p-tolyl)-2-methylbutene, to form 2,6-DMN in one step.
The present process provides for easy separation of by-products and the formation of alkyl-substituted indanes and indenes is minimized.
The present invention primarily relates to an improved method for manufacturing 2,6-DMN in one step instead of by a multistep synthesis, that is, to convert alkenylbenzene (alkylbenzene), 1-(p-tolyl)-2-methylbutene (1-(p-tolyl)-2-methylbutane) by dehydrocyclisation to 2,6-DMN. The catalysts described herein can, however, also be employed in conventional multistep-synthesis methods, although that application is not particularly preferred.
The present one-step reaction is carried out in the presence of a solid dehydrocyclisation catalyst comprising an essentially neutral support at an elevated temperature and ambient pressure in gaseous phase using a gas mixture as a carrier gas.
The catalyst can comprise activated carbon as such or a noble metal component, e.g. Rh or Pt, or a transition metal component, e.g. Cr, deposited on the support. In the presence of an essentially neutral catalyst system the reaction proceeds on the metal sites of the catalyst.
Experimental results obtained in connection with the present invention surprisingly indicate that activated carbon, also called activated charcoal and abbreviated xe2x80x9cACxe2x80x9d, exhibits high activity for the reaction of 1-(p-tolyl)-2-me-butene. The activated carbon can comprise a product of a quality conventionally suitable for use as a support for noble metal catalysts.
Three parameters are of particular importance for characterizing the carbon materials, viz. porosity, ash content/purity and acidity/alkalinity.
The activated carbons used in the present invention are porous, and the pore sizes depend on carbon type. Pores above 50 nm in width are generally called xe2x80x9cmacroporesxe2x80x9d, whereas pores between 2 and 50 nm are xe2x80x9cmesoporesxe2x80x9d, and pores below 2 nm in width are xe2x80x9cmicroporesxe2x80x9d. It should be noted that each of the carbon types comprises a wide range of pores with various sizes. However, according to a specific embodiment, it is preferred to use activated carbons in which at least 7% of the pores are mesopores or macropores. In particular, the fraction of meso- and macropores (2 nm less than dpore less than 50 nm and dpore greater than 50 nm) should be about 10 to 60%. It appears that the mass transfer of the reactants as well as the reaction of rather large compounds is significantly restricted in highly microporous materials. It is also possible that some of the pores of microporous materials become blocked when metal species are deposited on the surface thereof, whereas more mesoporous materials are better capable of retaining their pores open., but this is only a suggestion.
As mentioned above, the ash content of activated carbons also varies, and according to a specific embodiment it is preferred to use an activated carbon support having a low ash content. In particular, ash contents of less than 2% and preferably less than 1% are preferred. Low-ash carbons generally contain only small amounts of impurities such as sulphur and chlorine. A particularly preferred embodiment comprises a combination of low ash content ( less than 1%) and a mesoporous structure (fraction of meso- and macropores being larger than 15%).
Activated charcoal supports can also be characterized by their pH value as acidic, neutral or alkaline materials. Although the acidic carbons are active by themselves, according to the present invention, the performance of such a product is detrimentally influenced by its acidic nature and can be improved by decreasing the acidity.
According to a preferred embodiment, the acidity of the activated carbon is decreased by thermally decomposing the surface groups responsible for it. Thus, an acidic activated carbon can be subjected to a pretreatment carried out at an elevated temperature. Preferably the activated carbon is treated at temperatures in the range of 400 to 1000 xc2x0 C. The test results discussed below (cf. Table 5) indicate that the pretreated supports exhibited similar overall high activity for the reaction of 1-(p-tolyl)-2-me-butene as the support of example 19. In regard to the formation of 2,6-DMN, the selectivity of a pretreated acidic activated carbon support (AC(T)) was significantly better than that of the non-treated AC(T) support. Thus, a high temperature pretreatment eliminates acid sites from the surface and increases selectivity.
Aforesaid acidic sites can also be neutralised by using a metal, such as a metal of Group 1 to 4 and/or a metal of any of Groups 11 to 13. In particular, the catalyst can be neutralised with a suitable alkali metal, such as lithium, sodium or potassium. Thus, as evidenced by the examples below, in which acidic and neutral activated carbons were used as supports of rhodium catalysts, the neutralization of acidic sites on the surface of the carbon with potassium increases the selectivity of the catalysts. By contrast, for an activated carbon having an alkaline surface, no influence on the selectivity could be obtained.
It should be pointed out that the neutralization of the activated carbon by the metals can take place indirectly (=catalyst modification), if the metal species attach to the acidic sites of the carbon catalyst and thus changes the acidic character of the carbon. For the purpose of the present invention a carbon catalyst modified in the above manner is also called a neutralized or neutral carbon.
The neutral activated carbons can be used as supports for noble metal catalysts. The noble metal can be any noble metal having an activity in the present reaction, including noble Group VIII metals. Particularly preferred metals are rhodium, palladium and platinum. The noble metal can be deposited on the surface of the support by methods known per se, e.g. by impregnating or wetting the support with a solution or dispersion of a suitable metal or metal salt. The water or solvent is then removed and the dried material is optionally reduced to release the metal. The concentration of the noble metal is typically 0.01 to 50 wt-%, preferably about 0.1 to 20 wt-% and the metal particles are about 0.1 to 100 nm, preferably about 1 to 30 nm in diameter, in particular about 1 to 10 nm.
The selectivity of supported noble metal catalysts can be further improved by increasing the dispersion of the catalytically active species on the surface of the supports. It appears that meso- and macroporous activated carbon supports provide a better dispersion than microporous supports. A preferred embodiment comprises using a mesoporous activated carbon having a large number of surface binding sites, e.g. comprising acid groups. By using a support having many surface binding sites, the dispersion of the metal can be improved and the formation of aggregates reduced. After the deposition of the metal, the remaining acidic sites are then neutralized.
The benefial (neutralizing) influence of alkali metals was already discussed above. The alkali metal will work as promoter of the catalysts, and contribute to the activity thereof.
It is also possible to promote supported noble metal catalysts with ions of alkaline earth metals, such as magnesium, barium, calcium, or with ions of transition metals, such as copper, zinc, zirconium or cadmium or with boron. Although we do not wish to be bound to any specific theory, it appears that, e.g., zinc ions on the surface of Rh effectively blocks the large ensembles of Rh, and thus results in site isolation of surface Rh.
As the results given below show, in regard to the formation of 2,6-DMN, the selectivity of both the Ba and Zn promoted catalysts were significantly higher than those of their non-promoted counterparts. Thus, all the aforesaid promoters, such as potassium, barium and zinc, are beneficial in terms of the selectivity of the desired product, 2,6-DMN. The present invention also provides a novel heterogeneous catalyst comprising chromium on activated carbon. This catalyst can be used as a dehydrocyclisation catalyst. The catalyst can be prepared by conventional liquid phase or gas phase methods by depositing trivalent chromium ions on the surface of the support from suitable chromium salts. The liquid phase methods include impregnation or wetting of the carbon surface with solutions or dispersions of chromium salts in water or solvents, such as polar solvents.
The chromium loading on the support is 0.1 to 40 wt-%, preferably about 1 to 20 wt-%. The support can be any suitable activated carbon, but based on our experiences particularly preferred are meso- and macroporous activated carbons exhibiting 5 to 50% pores having diameters in the range of 2 to 50 nm and in excess of 50 nm. The surface of the support is neutral(ized).
The new chromium catalysts can be neutralized and promoted as explained above using thermal treatment, alkali metal ions, earth alkaline metal ions and transition metal ions. As the results given below show, the activity of a potassium promoted chromium-on-charcoal catalyst is comparable to that of potassium promoted chromium on alumina catalysts.
The dehydrogenation and cyclisation is carried out by contacting the starting material, such as 1-(p-tolyl)-2-methylbutene with the catalyst at an elevated temperature. The reaction temperature is in the range of 350 to 700 xc2x0 C., preferably 450 to 600 xc2x0 C. In particular the reaction temperature is below 550 xc2x0 C. At these temperatures the starting material will vaporize and the reaction is therefore carried out in gas phase.
Generally, the feed is conducted with a carrier gas into the reactor, in which the catalyst is kept at the reaction temperature. The contact time of the reaction is quite short, typically 3 seconds or less. Hydrogen is often used as a carrier because hydrogen has been found to inhibit fast deactivation of the catalysts. Hydrogen will also reduce the noble metal of the catalyst into elemental form. The problem with hydrogen is, however, that it causes hydrogenation of the starting material. According to the present invention this reaction can be significantly reduced by the use of either a mixture of hydrogen and argon or pure argon or nitrogen as a carrier instead of hydrogen. A particularly preferred embodiment comprises using a catalyst having lower hydrogenation activity than platinum, such as rhodium. Although the activity of rhodium catalysts decreases when the carrier gas is changed from hydrogen to the H2 (3%)/Ar mixture or pure argon, the selectivity of the dehydrocyclisation catalysts is significantly increased when the carrier gas is changed from hydrogen to a said mixture or to pure argon.
Furthermore, a mixture of hydrogen and argon, such as the above (H2 (3%)/Ar mixture) represents a particularly preferred embodiment for catalysts containing rhodium, because it can be used throughout the process, that is for the in situ reduction of the catalyst and as well as for carrier gas.
In summary, the use of a hydrogen/argon mixture will increase 2,6-dimethylnapthalene selectivity and decrease hydrogenation selectivity and/or indanes or indenes selectivity.